Process for pre-treating can materials for lacquering

ABSTRACT

The present invention comprises a process for pre-treating a steel strip intended for can making in the lacquring line in an electrolysis tank containing as the treating solution more than 50g/l of anhydrous chromic acid and sulfate ions in the amount of 2 to 5% of the anhydrous chromic acid, electrolytically treating the steel strip less than 0.2 sec. at a current density of 50 A/dm2 on the downward path in the tank, then dissovling the excessive part of attached hydrated oxide of chromium on the upward path and immediately applying lacquer. The chemical treatment process of the present invention is very easily connected to the lacquring line for steel strips, and thereby lacquered can materials possessing excellent anti-under-film corrosion and suitable for cans for carbonated beverages are supplied with decreased cost.

United States Patent [1 1 Umezono et al.

[ Sept. 3, 1974 PROCESS FOR PRE-TREATING CAN MATERIALS FOR LACQUERING [73] Assignee: Nippon Steel Corporation, Tokyo,

Japan 221 Filed: Mar. 7, 1973 211 App]. No.: 338,957

[30] Foreign Application Priority Data Primary Examiner-John H. Mack Assistant Examiner-R. L. Andrews Attorney, Agent, or FirmToren, McGeady and Stanger [5 7] ABSTRACT The present invention comprises a process for pretreating a steel strip intended for can making in the lacquring line in an electrolysis tank containing as the treating solution more than 50g/l of anhydrous chromic acid and sulfate ions in the amount of 2 to 5% of the anhydrous chromic acid, electrolytically treating the steel strip less than 0.2 sec. at a current density of 50 A/dm on the downward path in the tank, then dissovling the excessive part of attached hydrated oxide of chromium on the upward path and immediately applying lacquer. The chemical treatment process of the present invention is very easily connected to the lacquring line for steel strips, and thereby lacquered can materials possessing excellent anti-under-film corrosion and suitable for cans for carbonated beverages are supplied with decreased cost.

1 Claim, 3 Drawing Figures FIG] IPAIENIEUSEP 31914 v A 318-33483 Slim- 80? 2 DR 80 /dm L.S. 600mm Ba'ih Temperature 45C Deposition Efficiency of Me'iuiiic Chromium Second Time of Eiectro lysis good Estimation Metallic Chromium Amount /dm The present invention relates to a process for producing lacquered steel strip for can materials on a lacquering line comprising placing an electrolytic treating tank before the lacquering section and applying a simple but effective chemical treatment in the tank followed by immediate lacquering.

The primary object of the present invention is to provide can materials, especially those suitable for containing carbonated beverages which avoid the underfilm corrosion which is often present with cans for carbonated beverages. I

The second object of this invention is to provide cheap can materials especially suitable for cans for carbonated beverages by applying to steel strip materials, a chemical treatment which is effective especially against under-film corrosion in an electrolysis tank prior to lacquering and by then lacquering immediately thereafter.

Another object of this invention is to provide a process in which a chemical treatment is inexpensively and easily combined with the lacquering line of steel strip materials. A further object of the present invention is to provide a process of chemical treatment which is very suitable to pre-treatment for high speed lacquermg.

Tin plates are extensively used as the base material for making food containers because tin reacts as an effective anode to prevent the corrosion of the steel substrate of the containers which is in contact with various foods. However, in some food containers, especially those used for carbonated beverage, tin is normally cathodic to the exposed steel base and is unable to prevent the local corrosion or perforations of the steel substrate, so that cans lacquered on the inside are used to prevent corrosion of the steel substrate for containers of such foods.

Since the organic coating films protect the steel substrate against the corrosion on the inside of the cans it is necessary to use tin plate.

For the above reason, directly lacquered steel strips were used for can materials, but they were not commercially used because of problems with the under-film corrosion until the so-called TFS (tin-free steel) was developed. By under-film corrosion is that corrosion which is developed on the inside wall of lacquered cans on the interface of the lacquered film and the base steel starting from tiny defects or mars on the lacquered film inevitably formed on lacquering or in the process of can making. It results in peeling of the lacquered film and loss of protection against corrosion. Carbonated beverages, which are likely to promote the under-film corrosion, give rise to several problems with such cans.

The can material called TFS which has been practically used as a can material mostly for carbonated beverages has two coating layers on the surface of the base steel, namely, a lower layer of metallic chromium and an upper layer of hydrated oxide of chromium. Related patents include Japanese Patent No. 308,065, Patent Publication Sho 43-2768 (US. Pat. No. 3,296,100), Patent Publication Sho 46-6323 and Patent Publication Sho 46-19522. This material has been known for its excellent lacquer adhesion of paint and corrosion resistance of the lacquered surface.

In the production of TFS it is assumed that the product is. shipped without lacquering, and therefore rust prevention during long term storage and during transportation is an important factor. Therefore, a considerable amount of metallic chromium and hydrated oxide of chromium must be coated on the steel substrate, which requires another production line separate from the lacquering line, troublesome treatment and an enormous set of equipment. This leads inevitably to high prices of the lacquered can material prepared by the mentioned process.

On the other hand, can materials have been lacquered in a lacquering line for individual sheets, wherein each lacquered sheet is cured on a chain conveyor with wickets in an oven. Although the production speed is low owing to the limitation of the equipment, the process is suitable to production of small quantities of diverse can materials, that is, for example, containers of different design for different use; Consequently the lacquering costs are high and as the demand of cans for carbonated beverages has increased in recent years, cheaper materials for lacquered cans are needed.

In connection with the demand of can materials for carbonated beverages, the present invention is based on the technical thought that the steel strip is submitted to continuous chemical treatment as a pre-treatment in the lacquering line followed by continuous lacquering of the steel strip coil to improve the production speed and thereby to permit cheap and large scale supply of lacquered can materials. Moreover, if the abovementioned pre-treatment is adopted in the lacquering line, it is not necessary to produce the anti-rusting property of the steel surface before the lacquering step, though the anti-underfilm corrosion property of the lacquered surface is obtained after the lacquering.

Namely, the present invention comprises providing an electrolysis tank for pre-treating the steel strip in the lacquering line, the tank containing as a treating solution, more than SOg/l of anhydrous chromic acid and sulfate ions in an amount of 2 to 5% of the anhydrous chromic acid, electrolytically treating the steel strip for less than 0.2 sec. at a current density of 50 A/dm on the downward path in the tank, then dissolving the excessive part of the attached hydrated oxide of chromium on the upward path and immediately applying lacquer. With this process thus adopted, the chemical treatment process is very easily connected to the lacquering. line for steel strip, and thereby lacquered can materials excellent in the anti-under-film corrosion and suitable to cans for carbonated beverages can be supplied with less cost.

The features of the present invention will be described in detail referring to the attached drawings.

FIG. 1 illustrates the operation line to be used in the process of this invention. However, the invention should not be limited by what is shown in the figures.

FIG. 1 explains an example of the process employed in the present invention. FIG. 2 shows the relation between the time of electrolysis and the efficiency of deposition of metallic chromium and FIG. 3 shows the relation between the extent of under-film corrosion and the amount of metallic chromium.

In FIG. 1, (l) is a pay-off reel of cold reduced steel strip (black plate coil), (2) is the uncoiled steel strip and (3) shows the accumulator on the inlet side.

(4) is an electrolysis tank where a pair of anodes (5) are installed on the downward path on the inlet side. (6) isan apparatus for removing the electrolysis solution attached on the strip surface, such as, asqueezing roller and air wipers installed on the upward path above the electrolysis tank. The steel strip is dried in the dryer (7) and is lacquered with the coaters (8) while the surface retains activity. (9) is a curing oven, (10) is cooling equipment,.(1l) is an accumulator on the outlet side and the lacquered steel strip is recoiled on a recoiler (12) and is supplied as material for making cans.

A steel strip (2), cold reduced, anealed and tempered, is electrolytically treated in an electrolysis tank (4). The de-greasing and the acid pickling of the steel strip prior to the electrolysis are not necessary for a steel strip which has been processed in the ordinary process. However, for steel strip, the surface of which is dirty with rolling oil used during the tempering, it is preferably submitted to the de-greasing and the pickling before electrolysis. These treatments do not limit the use of thepresent invention.

A pair of anodes (5)'are installed on the downward path in the electrolysistank.(4), but not on the upward path.

A treating solution containing more than 50 g/l of anhydrous chromic acid and 2 5% as much sulfate ions as anhydrous chromic acid is placed in the tank (4). The steel strip (2) is treated as cathode on the downward path at the current density larger than 50 A/dm for less than 0.2 sec., and then non-electrolytically treated on the upward path to remove, by dissolution, the excessive amount of hydrated oxide of chromium over the controlled amount. Thus, a base layer with a thickness of 0.05 0.5 mgldm of metallic chromium and an upper layer of a verysmall amount of hydrous chromiumthat is" below 0.05 mg/dm as chromium are formed on the steel strip (2).

The treating solution of the present invention belongs to those solutionswhich employ highconcentration anhydrous chromic acid and sulfate ions. The reason why the concentration of anhydrous chromic acid islimited to above 50 g/! is asfollows:

In a treating solution containing less than 50 g/l of anhydrous chromic acid, the deposition efficiency of metallic chromium remains insufficient, and especially when the electrolysis in the tank is followed by a subsequent high speed lacquering line (e.g. 200 600 feet/- min.) deposition of metallic chromium does not amount to the predetermined quantity. On the other hand, an abundance of hydrated oxide of chromium is formed which makes it difficult in the dissolving treatment on the upward path to control the hydrated oxide of chromium to the predetermined small amount, leading necessarily to an ugly surface of the products after lacquering. g

In this respect, however, Patent Publication Sho 43-2768 (U.S. Pat. No. 3,296,100), referred, to above, usesless than 50 g/l of anhydrous chromic acid and thereby has an'object to produce an amount of hydrated oxide of chromium sufficient to secure the rust resistance by covering pin holes on the metallic chromium layer with the said oxide. Therefore, this invention is entirely different from the present invention with respect to the amount of anhydrous chromic acid.

For a higher concentration of anhydrous chromic acid, it notonly becomes difficult to obtain the necessary amount of high quality hydrated oxide of chromium, but also the amount of chromic acid remaining on the surface after the electrolysis and removing treatments increases to such an extent that the tone after lacquering may be spoiled. For these reasons, the concentration may be as high as 100 g/l.

The amount of sulfate ions is limited to 2 5% of anhydrous chromic acid. For a concentration below 2%, more hydrated oxide of chromium is formed and the dissolving rate on the upward path is decreased, and therefore it is difiicult to control hydrated oxide of chromium to the pre-determined amount. For a concentration exceeding 5%, it is difficult to produce the pre-determined amount of metallic chromium on the downward path of the electrolysis.

, The current density should be 50 A/dm or more, be-

cause the deposition efficiency of metallic chromium decreases for the current density below 50 A/dm The time duration of electrolysis on the downward path is 0.2 see. This was done according to the present inventors conclusion that the deposition efficiency of metallic chromium is excellent in the electrolysis treatment for periods as short as less than 0.2 sec., as FIG. 2, which includes an example thereof, demonstrates.

As for the electrolysis time, Patent Publication Sho 46-2768 (US. Pat. No. 3,296,100) referred to above andPatent Publication Sho 46-19522 adopted more than about 3 sec. and more than about 0.5 sec., respectively. However, a considerably shorter time of electrolysis is adopted in the present invention and a high efficiency of deposition of metallic chromium is obtained.

' sec. permits the electrolysis treatment to be conducted in a single tank with less cost and has allowed continuous treatment of steel strip in combination with the lacquering line.

Furthermore a high efficiency in deposition results in remarkable decrease in the cost for electric power.

The steel strip electrolytically treated in the electrolysis tank (4) is submitted to the dissolution treatment on the upward path of the electrolysis tank and then the electrolysis solution attached on the surface is removed by the electrolysis solution remover (6) installed on the upward path above the tank.

The electrolysis solution remover contains pairs of squeezing rollers, air wipers or a combination thereof. When a steel strip has left the electrolysis tank, it is necessary to reduce the amount of chromic acid remaining on the surface as much as possible. Squeezing rollers are apt to produce irregularity in squeezing, which spoils surface tone after lacquering and deteriorates the quality especially at those areas where a large amount of, chromic acid remains. If the removing is doneonly with squeezing rollers, those provided with convex crowns of hard rubber should be used and pressure as high as 5 kg/em should be applied to remove the electrolysis solution.

The most desirable device for removing the electrolysis solution is a combination of squeezing rollers and air wipers. Most of i the solution is removed with the squeezing rollers from the surface of the steel strip, and

then the remainder of the solution is uniformly and completely removed with the air wipers. If necessary, rinsing equipment can be installed instead of or in combination with those apparatus, but this installation does not limit the use of the present invention.

The steel strip, when it leaves the dryer (7), is coated with two layers on the surface, a base layer of metallic chromium as thick as 0.05 0.5 mg/d m and an upper layer of hydrated oxide of chromium'thinner than 0.05 mg/dm.

The amount of hydrated oxide of chromium, which is controlled on the upward path by the nonelectrolytic treatment in the specified treating solution, is restricted in the range described above for the following reason:

As has been described, the function of hydrated oxide of chromium in previous TFS is to cover pin holes in the base layer of metallic chromium and thereby to improve the rust resistance of the unlacquered surface. For that purpose, Patent Publication Sho 46-19522, for example, claims the amount of hydrated oxide of to 0.053 0.216 mg/dm (as Cr).

In the present invention, however, the electrolytic treatment as the pretreatment before lacquering is immediately followed by lacquering, so that the electrolytically treated steel strip is required-to have little rust resistance and such an amount of hydratedoxide of chromium is considered sufficient as to secure the lacquerability. Investigations of the present inventors on this subject revealed that hydrated oxide of chromium below 0.05 mg/dm (as Cr) issufficient to meet the mentioned purpose.

had been cold rolled, annealed and tempered, was directly treated cathodically under the following condition:

Bath 60 g/l CrO 1.2 g/l H 50 Temperature 45C Current density on cathode 80 A/tlm Time of electrolysis 0.06 sec.

After the electrolysis treatment, the surface of the specimen was squeezed with rollers under applied pressure of 8 kg/cm to remove the solution on the surface.

EXAMPLE 2:

Bath 60 5/] 00,

v I 2.0 g/l H,so,

Temperature 45C Current density on cathode' 80 A/dm Time of electrolysis 0.] sec.

The electrolysis solution was removed first by roll squeezing then with air wipers.

This is seen in Table 2. Further, if hydrated oxide of chromium is applied in the amount exceeding-0.05 mg/dm (as Cr), uniformity may be lost locally and therefore irregular luster may occur.

The steel strip (2), thus treated electrolytically and dried, is then lacquered immediately in the coater-(l0) while the surface still retains activity. The process that follows to obtain the product is shown in the drawings.

An overall process which combines the process where a steel strip is either plated with chromium or treated with chromic acid with the process of electrodeposition lacquering is a well known process as in Patent Publication Sho 46-21835 (U.S. Pat. No. 3,558,460). The invention claims that electrodeposition lacquer on a chromium plated base or on a chromic acid treated base results in a greater amount of electrodeposited lacquer film per unit time relative to those obtained when electrodeposition lacquering is applied to an otherwise treated surface.

On the other hand, the present invention does not intend to produce better workability as a result of electrodeposition lacquering, but to provide a method for combining the processes of electrolytical treatment and lacquering using an inexpensive and simple installation by specifying the composition of the electrolytic treatment solution and condition of the electrolysis, and further to provide cheap lacquered steel plates for making cans by specifying each process incorporated in the lacquering line. These are not described at all in the cited art. Therefore, the cited art and the present invention belong to different categories.

EXAMPLE 1:

Low carbon steel strip of 0.28mm thickness, which EXAMPLE 3:

Bath g/l CrO 1.6 g/l H 50, Temperature 45C Current density on cathode 60 A/dm Time of electrolysis 0.1 sec.

The elelctrolysis solution was removed by roll squeezing followed by applying air wipers.

Specimens treated according to Examples 1 to 3, respectively, were lacquered, immediately after being dried, with epoxyphenol lacquers (DIC 83-088, DIC 5 l-O55)'bymeans of a roll coater and cured, and thereafter'were tested with respect to the lacquer adhesion and the corrosion resistance of the lacquered materials.

Table 1 shows the amounts of metallic chromium, amounts of chromium in the hydrated oxide of chromium and amounts of applied lacquer of the materials obtained in Examples 1 to 3.

Table 2 shows results of tests specified in the following (1) through (4) applied to lacquered materials of which had been treated in Examples 1 through 3.

1. Draw test into caps Lacquered samples were drawn into caps in the shape of 27 X 20mm and the lacquer adhesion was tested by means of peeling-off with cellophane tape. 2. Steam retort test The lacquered samples were drawn into square 5A cans, processed in a steam retort at 121 C (1.1 kg/cm for min. and thereafter the fabricated portions were examined.

3. Under-film corrosion test Lacquered samples were scratched on the surface in the'shape of the letter X with a razor blade and were immersed in an aqueous solution containing 1.5 wt. percent of citric acid and 1.5 wt. percent of sodium chloride and saturated with carbon dioxide, for 4 days at room temperature, and then the scratched portions were examined.

4. Immersion test in citric acid Lacquered samples were subjected to Ericksen cups of 5m high, were immersed in an aqueous solution containing 10 wt. percent of citric acid for hours at the room temperature and then the fabricated portion were examined.

As is shown in Table 2, specimens in Examples 1 through 3 exhibited excellent properties as material for making cans, and therefore it is believed that the present invention has made it possible to supply a cheap and useful material for can making.

Table 1 Metallic Chromium Lacguer coating weight chromium in hyd- 1 (mg/dm) rated -Outside Inside oxide of (DlC 50 (DIC 83 chromium 055) 088) (mgldm (mg/dm) (mg/dm Black plate (no chemical treatment 45 45 applied) Specimen in Example I 0.20 0.02l 45 45 Specimen in Example 2 0.29 0.036 45 45 Specimen in Example 3 0.18 0.030 45 45 Table 2 Draw test Steam Under-film Immersion into caps retort corrosion test in test test citric acid Black plate Slight Slight Whole Whole (no chemical peeling swelling lacquer lacquer 8 treatment at the of lacqfilm film applied) processed I uer film peeled peeled part at the fabricated part Specimen in No peeling No peeling No peeling No peeling Example I at all at all at all at all Specimen in Example 2 do. do. do. do. Specimen in Example 3 do. do. do. do.

What is claimed is:

1. In a process for pre-treating can materials wherein cold stretched, tempered steel strips are lacquered, the improvement which comprises electrolytically treating the strip prior to lacquering by passing it through an electrolytic treating tank in first a downward path and then an upward path, said tank containing an electrolytic treating solution of at least 50 g/l anhydrous chromic acid and 2 to 5% sulfate ions based on the amount of anhydrous chromic acid, the strip being treated as a cathode on the downward path at a current density of more than 50 A/dm for a time of less than 0.2 sec. of the treating time and dissolving the hydrated chromium oxide on the upward path, removing the electrolyte solution on the upward path above the solution level in the tank, whereby a base layer of metallic chromium having a thickness of 0.05 to 0.5 mg/dm and an upper layer of hydrated chromium oxide of less than 0.05 mg/dm' as chromium is formed on the strip and then drying and immediately lacquering the steel strip while the surface remains active. 

